The present invention relates to track pitch calibration systems, and more particularly to a track pitch calibration system that may be used in an optical storage system to accurately set and maintain a prescribed track pitch for data tracks placed on a record carrier used within the optical storage system.
An optical disk storage system of the type to which the present invention pertains comprises an optical drive into which a record carrier is inserted. Data is stored by marking the record carrier, such as a rotating disk, with a beam of radiant energy (typically a laser beam) that is modulated in some fashion by the data to be stored. To write or store data on the record carrier, the modulated beam is directed to and focused at a desired point on the surface of the record carrier and relative motion is created between this part and the record carrier. Where the record carrier is a disk, this relative motion is typically created by rotating the disk. As the disk rotates under the desired point, a "data track" is created by the marks made on the disk by the modulated beam. If the desired point at which the beam is focused is held stationary, a circular data track is created on the disk that is centered about the axis of rotation of the rotating disk. Additional data tracks, each concentric with the others, can be created by turning the write beam off, radially moving the point at which the write beam is focused to a new location, holding this point stationary at the new location, and turning the modulated write beam back on. Alternatively, if the point at which the write beam is focused is radially moved with respect to the disk as the modulated write beam makes marks thereon, a spiralling data track is created on the surface of the disk.
Whether the data tracks are concentric or spiralling, the available surface area on the disk is most efficiently used when the data tracks are spaced together as close as possible. The radial distance between adjacent data tracks is called the "track pitch". Accurately maintaining the track pitch at a desired value, especially where the track pitch must be kept small so as to efficiently make use of the storage space available on the disk, has presented a significant obstacle in the development of high storage capacity optical disk storage systems.
Data tracks are typically read by directing a read beam of radiant energy to a desired data track on the disk. This read beam typically has different parameters associated therewith than does the write beam (such as intensity and/or wavelength), thereby ensuring that the read beam does not mark the disk in the same manner as the write beam is designed to mark the disk. The read beam is either reflected from the surface of the disk, or passes therethrough (if the substrate of the optical disk is sufficiently transparent to allow the beam to pass therethrough), and the intensity of the read beam is modulated in accordance with the data marks that have been previously written in the data track by the write beam. The data marks typically comprise a sequence of reflectivity-high/reflectivity-low (or transmissivity-high/transmissivity-low) marks that modulate the reflected or transmitted read beam in accordance with the pattern of the stored data. Once modulated, the read beam is directed to a suitable optical detector where a modulated data signal is generated. The data is extracted from this signal during conventional demodulation techniques.
Whether the optical drive is reading or writing data, it is critically important to be able to set and maintain a prescribed track pitch. When writing data, the newly written data track must be spaced the prescribed track pitch distance from a previously written data track or other suitable reference. When reading data, the prescribed track pitch distance must be known so that the optical drive can direct the read beam to a desired track location on the surface of the disk as measured from a known location on the disk, such as an adjacent track. Moreover, if interchangeability of record carriers from optical drive to optical drive is to be preserved--a condition that must exist if the record carrier or disk is removable from the optical drive--the prescribed track pitch must be maintained from one record carrier to another, and from one optical drive to another. This is not an easy task to achieve given the numerous variations that naturally occur in the electrical and optical components used within any optical storage system.
To illustrate some of the difficulties encountered, when writing data tracks on a record carrier, the desired track pitch is typically achieved by positioning the write beam a prescribed distance from a reference beam that is following a previously written track. Hence, the goal is that the newly written track will be held at a constant distance from the next preceding already recorded data track. See U.S. Pat. No. 3,657,707 col. 11, lines 40-43. In order to maintain the prescribed distance between the two beams, however, some sort of control system must be used to keep the two beams separated the desired amount. While this goal is easily stated, the realization thereof is not easily achieved.
A significant first hurdle is to initially set the two beams so as to have the desired spacing therebetween. This is not a trivial task because of the nature of the elements involved. While galvonometer controlled mirrors and related optical components can be used to position one beam relative to another in response to a control signal, the initial value of the control signal must still be determined. Unfortunately, because of differences in the electrical and optical components from one optical drive to another optical drive, this control signal will not necessarily be the same for all optical drives.
Even assuming that the two beams can be initially spaced apart the prescribed distance, a difficulty arises in maintaining this distance. Both the electrical signals generated to set the distance, and the electromechanical elements used to respond to these signals, will not be perfectly stable over time and a range of environmental and other conditions. Moreover, the optical elements used within the drive to direct the beams to and from the surface of the record carrier will likewise exhibit some variations over time and environmental conditions, and especially will exhibit variations from optical drive to optical drive.
One approach for initially setting and maintaining a desired track pitch, as described in copending patent application Track Pitch Calibration System for use in Optical Storage Devices, Ser. No. 628,687, filed concurrently herewith, assigned to the same assignee as is this application, is to place a pair of calibration tracks having the desired track pitch therebetween on the disk or record carrier during the manufacture thereof. The track pitch of these calibration tracks is then optically measured when the record carrier is first inserted into the optical storage system (and at calibration intervals thereafter), in order to define an offset signal that defines the prescribed track pitch for that particular storage system, including the particular combination of electrical and optical elements found therein. This offset signal is thereafter used to set the desired spacing between the various beams used within the optical storage system to insure that the desired track pitch is maintained. Unfortunately, because the measurement of the track pitch of the calibration tracks is made optically, it is influenced by the presence or absence of data tracks adjacent to the tracks being measured. Hence, the calibration offset signal generated while measuring the pitch of the two calibration tracks will not be the same read offset signal needed to space two read beams apart the prescribed track pitch distance where there exists data tracks on both sides of the two tracks being read; nor will the calibration offset signal be precisely the same write offset signal needed to space a write beam from a read beam the prescribed track pitch distance when a data track is adjacent one side of the track being read, but no data track is adjacent to the other side of the track being written.
Hence, there is a need in the art for a track pitch calibration system that can initially set and maintain a desired track pitch in an optical storage system regardless of whether optical storage system is reading or writing data, and regardless of the number of data tracks found on the record carrier used with such a system (i.e., regardless of whether the record carrier is full, empty, or somewhere inbetween). The present invention addresses that need.